Doc, I look forward to your report when you solve the mystery. Seems like your chrony is one good tool to help in your investigation.
I wonder if you might try to include a lubed felt wad or even a fiber wad to assist as a gas check. Seems like there are a number of Walkers with groove diameters much larger than their chamber diameters. Also, I'd like to inquire as to the hardness of your lead ball used. Are you casting your own?